1. Field of the Invention
The present invention relates to an optical recording medium referred to as a next generation DVD (Digital Versatile Disc). In particular, the invention relates to an optical recording medium having-two or more information layers formed of a phase-change material and to a recording film material for the optical recording medium.
2. Description of the Related Art
For example, Blu-ray (trademark) discs (hereinafter abbreviated as BDs) have been proposed as next generation DVDs. For the BDs, an optical system is used which uses a recording-reproduction laser beam having a wavelength of 405 nm (blue) and an objective lens having a numerical aperture NA of 0.85 (λ/NA≦500 nm).
In optical disc drives for BDs or the like, a single-mode oscillation diode laser is used, and high frequency modulation for a reproduction beam is performed at a frequency of several hundred MHz in order to reduce laser noise during signal reproduction. Generally, the conditions for high frequency modulation are a frequency of 300 to 500 MHz, a Ratio of 3 to 8, and a pulse width of 200 to 400 psec.
For BDs, rewritable optical recording media have been proposed which have two or more information layers on one side. In such optical recording media, information layers other than an information layer (L0 layer) located farthest from a laser beam incident surface must be translucent information layers that are translucent to the wavelength of a recording-reproduction laser beam, in order to allow the laser beam directed to the L0 layer to pass therethrough. For example, in dual-layer optical recording media, an L1 layer other than the information layer (L0 layer) located farthest from the laser beam incident surface is a translucent information layer. Therefore, the reflectivity from the L1 layer is low, and the reflectivity from the L0 layer is also low because the laser beam is incident on and reflected from the L0 layer through the L1 layer. In such dual-layer optical recording media, the reflectivity is lower than that of single-layer optical recording media. Accordingly, the power of the reproduction laser beam must be increased to ensure a sufficient amount of light reaching a pickup.
Moreover, the laser beam used for BDs has a shorter wavelength than that used for DVDs, and an objective lens having a high NA is used in the BDs, whereby a spot size is reduced. Therefore, the energy density at the laser spot is very high.
Furthermore, as described above, since the L1 layer in the dual-layer optical recording media must be a translucent information layer, the thickness of a metal reflection film must be reduced. Therefore, the heat generated by the laser beam irradiated onto the recording film of the L1 layer is not sufficiently dissipated from the reflection film, so that the cooling rate in the L1 layer is lower than that in the L0 layer, i.e., the L1 layer has a slow-cooling structure.
Moreover, when the linear velocity during recording is increased to perform high-speed recording, i.e., when the rotation speed of the disc is increased, the pickup cannot easily follow grooves on the disc, and therefore servo control is not stabilized. Hence, when high-speed recording is performed, the reproduction power must be increased to stabilize the servo control.
Accordingly, in the translucent information layers in next generation DVDs having two or more information layers, recorded signals deteriorate due to the reproduction beam, so that the reproduction durability significantly deteriorates. This problem is caused by the following four main reasons: high reproduction power, high energy density at a small size spot, the slow-cooling structure in the translucent information layers, and high-speed recording.
Moreover, the high frequency modulation is performed on the reproduction beam. Therefore, the higher the Ratio (the ratio between the maximum power and the minimum power) in the high frequency modulation, the higher the peak power of the reproduction beam, so that the temperature increase in the portion irradiated with the reproduction beam becomes significant. Since a high-power reproduction beam under high frequency modulation at high Ratio is used in BDs, it is more difficult to achieve sufficient reproduction durability in the BDs than in DVDs.
In particular, in rewritable phase-change optical recording media, amorphous marks serving as recorded signals can be crystallized when irradiated with a high-power laser beam during reproduction, so that the recorded signals are likely to be lost. This problem is particularly significant in Sb-based eutectic phase-change materials containing Sb as a main component.
Conventionally, it has therefore been difficult to use an Sb-based eutectic material as the material for a recording film of a translucent information layer in rewritable multi-layer optical recording media for BDs.
When such an Sb-based eutectic material is used as the material for the recording film of the translucent information layer in the rewritable multi-layer BDs, a margin for recording strategy when recording is performed is reduced. Specifically, the following problems may occur:
With an Sb-based eutectic material, the crystallization speed of the recording film can be easily increased. However, a high-cooling rate is required when amorphous marks are formed by using a recording laser beam. If the cooling rate is not sufficiently high, recrystallization occurs during cooling after the melting of the recording film, so that the formation of the amorphous marks may be insufficient.
In view of the above, the cooling rate after the melting of the recording film must be increased by changing the structure of the medium to a rapid-cooling type structure or by narrowing the pulse width for forming the amorphous marks in the recording strategy.
However, in the translucent information layers in the multi-layer BDs, the thickness of the recording film or reflection film must be reduced in order to allow the recording-reproduction beam to pass therethrough. Therefore, the remaining heat is not dissipated sufficiently, and the translucent information layers have a slow-cooling structure in which the cooling rate is lower than that in a total reflection information layer (L0 layer). Accordingly, the formation of the amorphous marks may be insufficient.
Moreover, when a laser beam is generated, the required rise and fall time periods for the beam generation are approximately 1.5 nsec to approximately 3 nsec. Hence, if the pulse width of the laser beam is set shorter than the above beam generation time periods, the laser beam cannot be generated. Accordingly, the recording film is required to have the ability to allow recording with a pulse width larger than the above time periods.
However, with an Sb-based eutectic material, a high-cooling rate is required as described above. Therefore, when recording is performed by using a recording strategy with a large pulse width, recrystallization occurs, so that it is difficult to perform the recording correctly.
In order to solve the above problems, in Japanese Patent Application Laid-Open No. 2004-306595 and Japanese Patent No. 3899770, a lanthanoid element is added to a phase-change material. However, in these inventions, sufficient reproduction durability is difficult to be achieved at a high reproduction power in an optical system with λ/NA≦500 nm.